U.S. patent number 11,146,132 [Application Number 16/744,616] was granted by the patent office on 2021-10-12 for permanent magnet electric machine with variable magnet orientation.
This patent grant is currently assigned to GM Global Technology Operations LLC. The grantee listed for this patent is GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to Khwaja M. Rahman, Craig S. Ross, Shawn H. Swales, Goro Tamai.
United States Patent |
11,146,132 |
Swales , et al. |
October 12, 2021 |
Permanent magnet electric machine with variable magnet
orientation
Abstract
A permanent magnet electric machine (PM machine) includes a
rotor with rotatable magnets and a stator defining an air gap with
the rotor. An actuator rotates the rotatable magnets at
predetermined operating points through an angular distance
sufficient for changing magnetic pole orientations of the rotatable
magnets, and thus modifies magnetic flux linkage with stator
windings across the air gap. Fixed magnets may be arranged around a
circumference of the rotor. The actuator may be actively or
passively driven. Flux-shunting elements are optionally disposed in
the rotor to further modify the flux linkage. A gear set connected
to torque transfer elements may be driven by the actuator to rotate
the rotatable magnets. A vehicle includes drive wheels, a
transmission, and the PM machine. A method controls magnetic flux
linkage in the PM machine noted above.
Inventors: |
Swales; Shawn H. (Canton,
MI), Rahman; Khwaja M. (Troy, MI), Ross; Craig S.
(Ypsilanti, MI), Tamai; Goro (Bloomfield Hills, MI) |
Applicant: |
Name |
City |
State |
Country |
Type |
GM GLOBAL TECHNOLOGY OPERATIONS LLC |
Detroit |
MI |
US |
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Assignee: |
GM Global Technology Operations
LLC (Detroit, MI)
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Family
ID: |
1000005858142 |
Appl.
No.: |
16/744,616 |
Filed: |
January 16, 2020 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20200153299 A1 |
May 14, 2020 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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15860239 |
Jan 2, 2018 |
10581287 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02K
1/276 (20130101); H02K 21/028 (20130101); H02K
1/2766 (20130101); H02K 1/30 (20130101) |
Current International
Class: |
H02K
1/27 (20060101); H02K 1/30 (20060101); H02K
21/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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H11113199 |
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Apr 1999 |
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JP |
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H11355988 |
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Dec 1999 |
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JP |
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2013207943 |
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Oct 2013 |
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JP |
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Primary Examiner: Nguyen; Tran N
Attorney, Agent or Firm: Quinn IP Law
Parent Case Text
CLAIM OF PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of U.S. patent application Ser.
No. 15/860,239, which was filed on Jan. 2, 2018, is now allowed,
and is incorporated herein by reference in its entirety and for all
purposes.
Claims
What is claimed:
1. An electric machine, comprising: a rotor assembly including a
generally cylindrical rotor rotatable about a rotor axis; a stator
separated from the rotor by an air gap and bearing an electrically
conductive stator winding; a plurality of fixed magnets
circumferentially spaced about and rigidly mounted to the rotor;
and a plurality of rotatable magnets circumferentially spaced about
and movably mounted to the rotor, each of the rotatable magnets
being interleaved between a respective pair of the fixed magnets
and each having respective magnetic poles with a respective
north-south magnetic pole orientation, wherein each of the
rotatable magnets is rotatable on a respective center axis at a
center thereof to change the respective north-south magnetic pole
orientation thereof and modify a magnetic flux linkage with the
stator winding across the air gap.
2. The electric machine of claim 1, wherein each of the fixed
magnets is arranged in a respective V-shaped pair with a respective
one of the rotatable magnets.
3. The electric machine of claim 1, wherein each of the fixed
magnets has a respective center, the centers of the fixed magnets
and the center axes of the rotatable magnets being equidistantly
radially spaced from the rotor axis of the rotor.
4. The electric machine of claim 1, wherein a first of the
rotatable magnets is circumferentially spaced from and arranged in
a V-shaped pair with a first of the fixed magnets when the first
rotatable magnet is in an inactive flux-maintaining position and/or
an active flux-weakening position.
5. The electric machine of claim 1, wherein the fixed magnets are
spaced equidistant from one another about a circumference of the
rotor, and the rotatable magnets are spaced equidistant from one
another about the circumference of the rotor.
6. The electric machine of claim 1, wherein each of the rotatable
magnets includes a respective solid bar permanent magnet disposed
inside a respective rotatable plate, the rotatable plates being
rotatably mounted inside the rotor.
7. The electric machine of claim 1, further comprising an active or
passive actuator operatively connected to the rotatable magnets and
configured to selectively rotate the rotatable magnets at a
predetermined operating point of the electric machine to change the
respective north-south magnetic pole orientations and thereby
modify a level of the magnetic flux linkage.
8. The electric machine of claim 7, further comprising a controller
communicatively connected to the actuator and configured to output
thereto an electronic control signal, wherein the actuator is an
active actuator device configured to rotate the rotatable magnets
in response to the control signal from the controller.
9. The electric machine of claim 7, wherein the actuator is a
passive actuator device comprising one or more spring elements
coupled to the rotatable magnets and configured to bias the
rotatable magnets in a first rotational direction below a threshold
acceleration force of the rotor, and to permit rotation of the
rotatable magnets in a second rotational direction above the
threshold acceleration force.
10. The electric machine of claim 7, further comprising a plurality
of torque transfer elements driven by the actuator, wherein each of
the rotatable magnets includes a permanent magnet mounted to a
respective one of the torque transfer elements.
11. The electric machine of claim 10, further comprising a gear set
with a plurality of pinion gears each meshingly engaged with a sun
gear and connected to a respective one of the torque transfer
elements, wherein the actuator selectively rotates the sun gear to
thereby drive the torque transfer elements via the pinion
gears.
12. The electric machine of claim 11, wherein the actuator is a
passive actuator comprising: a spring element connected to the sun
gear; and a plurality of flyweights each connected to a respective
one of the pinion gears, the flyweights being configured to cause
rotation of the pinion gears in response to acceleration forces of
the rotor in excess of a return force of the spring element.
13. The electric machine of claim 1, further comprising a
flux-shunting element movably mounted to the rotor and having
non-uniform magnetic permeability properties on opposite halves of
the flux-shunting element.
14. A vehicle comprising: a vehicle body with a plurality of road
wheels; a transmission with transmission input and output members,
the transmission output member being drivingly connected to the
road wheels; and a permanent magnet (PM) electric machine
configured to generate motor torque and drivingly couple to the
transmission input member to thereby deliver the motor torque to
the transmission, the PM electric machine including: a rotor
assembly including a rotor shaft and a generally cylindrical rotor
coaxially surrounding and drivingly coupled to the rotor shaft; a
stator coaxial with and separated from the rotor assembly by an air
gap, the stator bearing a plurality of electrically conductive
stator windings; a plurality of fixed magnets circumferentially
spaced about and rigidly mounted to the rotor; a plurality of
rotatable magnets circumferentially spaced about and movably
mounted to the rotor, each of the rotatable magnets being
interleaved between a respective pair of the fixed magnets and each
having respective magnetic poles with a respective north-south
magnetic pole orientation; and a passive or active actuator
drivingly connected to and configured to selectively rotate the
rotatable magnets at a predetermined operating point of the PM
electric machine to change the respective north-south magnetic pole
orientations and thereby modify a level of magnetic flux linkage
with the stator windings across the air gap.
15. A method for assembling an electric machine, the method
comprising: providing a rotor assembly with a generally cylindrical
rotor; mounting a stator adjacent the rotor assembly with the
stator separated from the rotor by an air gap, the stator bearing
an electrically conductive stator winding; rigidly mounting a
plurality of fixed magnets to the rotor with the fixed magnets
circumferentially spaced from one another about the rotor; and
rotatably mounting a plurality of rotatable magnets to the rotor
with the rotatable magnets circumferentially from one another about
the rotor, each of the rotatable magnets being interleaved between
a respective pair of the fixed magnets and each having respective
magnetic poles with a respective north-south magnetic pole
orientation, wherein each of the rotatable magnets is rotatable on
a respective center axis at a center thereof to change the
respective north-south magnetic pole orientation thereof and modify
a magnetic flux linkage with the stator winding across the air
gap.
16. The method of claim 15, wherein mounting the fixed and
rotatable magnets to the rotor includes arranging each of the fixed
magnets in a respective V-shaped pair with a respective one of the
rotatable magnets.
17. The method of claim 15, wherein centers of the fixed magnets
and the center axes of the rotatable magnets are equidistantly
radially spaced from a rotor axis of rotation of the rotor.
18. The method of claim 15, wherein a first of the rotatable
magnets is circumferentially spaced from and arranged in a V-shaped
pair with a first of the fixed magnets when the first rotatable
magnet is in an inactive flux-maintaining position and/or an active
flux-weakening position.
19. The method of claim 15, wherein the fixed magnets are spaced
equidistant from one another about a circumference of the rotor,
and the rotatable magnets are spaced equidistant from one another
about the circumference of the rotor.
20. The method of claim 15, wherein each of the rotatable magnets
includes a respective solid bar permanent magnet disposed inside a
respective rotatable plate, the rotatable plates being rotatably
mounted inside the rotor.
Description
INTRODUCTION
Electric machines in the form of traction motors and electric
generators are used to generate torque in a wide variety of
electromechanical systems. Electric machines typically include a
rotor and coupled rotor shaft that are concentrically positioned
with respect to a stator. The rotor shaft rotates when the electric
machine is energized by a high-voltage power supply, such as an
inverter and multi-cell battery pack. Motor torque transmitted by
the rotor shaft may be used to perform work in the
electromechanical system, such as for generating electricity,
cranking and starting an engine, or propelling a vehicle.
In a permanent magnet-type electric machine, or "PM machine",
permanent magnets constructed of a rare earth material are
surface-mounted to or embedded within the structure of the rotor. A
core of the stator defines multiple slots that are individually
wound with conductive wires or bars to form electrically conductive
stator windings. The stator windings are sequentially energized by
a polyphase input voltage to produce a rotating electromagnetic
field. The rotating electromagnetic field in turn interacts with
the permanent magnetic fields of the rotor. Such field interaction
occurs in a magnetic circuit in which magnetic flux paths extend
across a small air gap from the rotor into the stator. Motor torque
from the PM machine is thus generated by the interaction of the
rotor's magnetic field, which is created by the magnets, with the
stator's magnetic field as created by external control of the input
voltage.
SUMMARY
A permanent magnet-type electric machine ("PM machine") is
disclosed herein in which magnetic north-south pole orientations of
a set of permanent magnets are actively or passively adjusted such
that the PM machine achieves variable reluctance and flux
characteristics at different operating points. The controlled
change in the magnetic pole orientations enables magnetic flux
between the rotor and stator to be modified in real time, for
instance with magnetic flux between the rotor and stator being
reduced above a threshold rotational speed and below a threshold
output torque of the rotor.
Relative to an internal combustion engine, a PM machine is a
relatively efficient generator of torque under
low-speed/high-torque conditions. Such conditions may be present
when motor torque is directed to drive wheels of a motor vehicle to
accelerate from a standstill. At higher speeds, however, motor
losses increase due to flux energy loss caused by the rotating
magnets and a requirement for field-weakening.
High-speed/low-torque operating points may therefore result in
flux-related losses that, if reduced, would benefit a system
employing such a PM machine. The disclosed PM machine is intended
to provide a possible solution to this particular phenomenon
without compromising the structural integrity and packaging
requirements of the electric machine.
In an example embodiment, the PM machine includes a rotor that is
splined or otherwise connected to a rotor shaft. The PM machine
also includes a stator and an actuator. Rotatable magnets are
connected to the rotor. The stator is spaced art from the rotor by
an air gap such that magnetic flux paths exist between the stator
and the rotor.
The actuator selectively rotates the rotatable magnets, either
actively or passively, at one or more predetermined operating
points of the PM machine. Rotation occurs through an angular
distance that is sufficient for changing the north-south magnetic
pole orientations of the rotatable magnets to a desired extent,
e.g., up to 180.degree. or more depending on the application.
Rotation either clockwise or counterclockwise may be possible
depending on the operating mode, e.g., while motoring or in a
regenerating mode. The reluctance, and thus the magnetic flux paths
between the rotor and the stator, is modified in this manner, i.e.,
by modifying a level of magnetic flux linkage with stator windings
across the air gap.
The rotor may optionally include fixed magnets arranged around a
circumference of the rotor in alternating north-south magnetic pole
orientations. The angular distance noted above is sufficient for
counteracting magnetic flux from the fixed magnets. The rotor has
an outer diameter defined by an outer perimeter wall and an inner
diameter defined by an inner perimeter wall. A respective one of
the rotatable magnets may be positioned adjacent to one or more of
the fixed magnets at a position adjacent to the inner perimeter
wall.
A respective one of the rotatable magnets may be positioned
adjacent to a respective one of the optional fixed magnets to form
a pole pair therewith. Optionally, a flux-shunting element may be
disposed at each pole pair, either as a separate component from the
rotatable magnets or a part thereof. The flux-shunting element
possibly having a non-uniform material composition and/or shape and
an adjustable orientation, such that non-uniform magnetic
permeability properties exist on diametrically-opposite halves of
the flux-shunting element. Orientation of the flux-shunting element
may be used to further affect the flux between the rotor and
stator, with the flux-shunting element possibly including another
magnet as part of its construction.
The actuator may be configured to selectively rotate the rotatable
magnets at one or more predetermined operating points of the PM
machine. The predetermined operating point(s) may be predetermined
torque-speed operating points of the PM machine, with the
predetermined torque-speed operating points corresponding to one or
more rotational speeds of the rotor exceeding corresponding
threshold speeds and motor torques of the rotor that are less than
corresponding threshold motor torques.
A controller may be in communication with the actuator, with the
actuator rotating the rotatable magnets, and possibly the optional
flux-shunting elements, in response to a control signal from the
controller. The actuator may include a spring mechanism or other
passive element configured to bias the rotatable magnets/optional
flux-shunting elements in a first predetermined rotational
direction below a threshold acceleration force of the rotor, and to
enable rotation of the rotatable magnets/optional flux-shunting
elements in a second predetermined rotational direction above the
threshold acceleration force of the rotor.
A vehicle is also disclosed herein having drive wheels, a
transmission, and a PM machine. The transmission has an input
member and an output member, with the output member being connected
to the drive wheels. The PM machine delivers motor torque to the
input member and is configured as described above.
A method is also disclosed herein for controlling flux in a PM
machine. The method includes connecting a plurality of rotatable
magnets to a rotor such that the rotatable magnets have a
respective north-south magnetic pole orientation. The method also
includes positioning a stator with respect the rotor such that the
stator circumscribes the rotor and defines an air gap in
conjunction therewith. Additionally, the method includes
selectively rotating the rotatable magnets, via an actuator, at one
or more predetermined operating points of the PM machine, through
an angular distance sufficient for changing the respective
north-south magnetic pole orientations and thereby modifying a
level of magnetic flux linkage with stator windings across the air
gap.
The above-noted features and advantages, and other features and
advantages of the present disclosure, will be readily apparent from
the following detailed description of the embodiments and best
modes for carrying out the disclosure when taken in connection with
the drawings and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of an example vehicle having a
permanent magnet-type electric machine (PM machine) with variable
magnetic pole orientations as set forth herein.
FIG. 2 is an example motor speed-torque plot, with motor torque
depicted on the vertical axis and motor speed depicted on the
horizontal axis.
FIGS. 3A, 3B, and 3C are schematic plan view illustrations of a
rotor usable with the PM machine of FIG. 1 according to example
configurations.
FIGS. 4A and 4B are schematic plan view illustrations of an
alternative configuration of the rotor shown in FIGS. 3A-3C.
FIG. 5 is a schematic plan view illustration of a possible active
embodiment for rotating a permanent magnet within the PM machine of
FIGS. 3A-C and 4A-4B.
FIG. 6 is a schematic side view illustration of a passive
alternative embodiment to that which is shown in FIG. 5.
FIG. 7 is a schematic plan view illustration of an optional
flux-shunting element usable with the PM machine of FIG. 1.
FIGS. 8A and 8B are schematic plan view illustrations of a possible
flyweight-based passive variation of the configuration shown in
FIG. 5.
The present disclosure is susceptible to various modifications and
alternative forms, and some representative embodiments have been
shown by way of example in the drawings and will be herein
described in detail. It should be understood, however, that the
novel aspects of this disclosure are not limited to the particular
forms illustrated in the appended drawings. Rather, the disclosure
is to cover all modifications, equivalents, combinations,
sub-combinations, permutations, groupings, and alternatives falling
within the scope and spirit of the disclosure as defined by the
appended claims.
DETAILED DESCRIPTION
Referring to the drawings, wherein like reference numbers refer to
like components, FIG. 1 depicts an example motor vehicle 10 having
an electric powertrain 12. The electric powertrain 12 includes a
high-voltage battery pack (B.sub.HV) 14 having a plurality of
battery cells 14C. The battery pack 14 is electrically connected to
a permanent magnetic-type electric machine (M.sub.E) 16,
hereinafter referred to as the PM machine 16, constructed with a
variable magnetic orientation as set forth below with reference to
FIGS. 3A-8B. In certain embodiments, the PM machine 16 may be used
as an electric traction motor aboard the motor vehicle 10 to
generate motor torque (arrow T.sub.M) at a level sufficient for
propelling the vehicle 10, as an electrical generator, and/or for
performing other useful work.
Applications of the PM machine 16 shown in FIG. 1 are not limited
to mobile or vehicular applications in general, or to motor vehicle
propulsion applications in particular. Those of ordinary skill in
the art will appreciate that the attendant benefits of the
disclosed PM machine 16 when constructed as described below with
reference to FIGS. 3A-8B may be extended to stationary and mobile
applications that rely on the use of the motor torque (arrow
T.sub.m), particularly under high-speed/low-torque operating
conditions of the PM machine 16.
As a foundational basis for the present disclosure, and referring
briefly to the example torque-speed operating plot 13 of FIG. 2, it
is recognized herein that PM machines typically experience
relatively high losses as a percentage of useful power when
operated at a high motor speed (N.sub.r) and a low load or motor
torque (T.sub.m), with motor speed (N.sub.r) depicted on the
horizontal axis and stated in revolutions per minute (rpm) and
motor torque (T.sub.m) depicted on the vertical axis and stated in
Newton-meters (N-m). A representative contour is shown in FIG. 2
indicated at zone Z wherein magnetic flux is excessive and flux
reduction would lower losses in the PM machine, an inverter, and
associated power electronics.
The zone (Z) is indicative of the relatively large fixed magnetic
fields emanating from magnets of a typical PM machine, e.g., large
rectangular bar magnets embedded in opposing slots to form
north-south pole pairs around a circumference of a rotor of such a
PM machine. That is, a magnetic circuit exists between a rotor and
a stator of a typical PM machine, across a small air gap there
between. Flux may be generated in excess when, as in zone Z, the
generated excess magnetic flux is detrimental to motor efficiency
and produces higher losses. A solution proposed herein is the
addition of flux leakage in main flux paths of the magnetic
circuit, such that the excess magnetic flux can be reduced in zone
Z of FIG. 2.
The PM machine 16 of FIG. 1 is thus configured with a variable
magnetic pole orientation intended to improve efficiency of the PM
machine 16 by reducing such excess flux. This goal may be
accomplished by selectively varying magnetic north-south pole
orientations within the PM machine 16. Such modifications in turn
modify the above-noted magnetic flux paths. Various embodiments for
implementing the disclosure are described in further detail below
with reference thereto FIGS. 3A-8B, including some embodiments in
which the modifications to the flux paths are assisted by
interposing optional "flux-shunting" elements having disparate
magnetic permeability characteristics due to non-uniform geometry
and/or materials of construction, particularly with respect to
diametrically opposite sides or halves of such flux-shunting
elements.
Returning to FIG. 1, the battery pack 14 may be optionally
recharged via an offboard charging station 11, for instance a
direct current (DC) fast-charging station as shown, with the charge
possibly delivered directly to the battery pack 14 using an applied
DC voltage (VDC) via a charge coupling device 20, to a DC voltage
bus 22 via a voltage regulator 24 having one or more internal
semiconductor and/or mechanical switches S, or via an available
alternating current charging system (not shown).
The electric powertrain 12 also includes a power inverter module
(PIM) 18 that is electrically connected to the battery pack 14 via
the DC voltage bus 22. Internal semiconductor switches (not shown)
of the PIM 18 are automatically controlled via pulse width
modulation or other desired switching techniques in order to
generate an alternating current (AC) output voltage suitable for
energizing the PM machine 16. An AC voltage bus 40 is used to
electrically connect the PIM 18 to the individual phase windings of
the PM machine 16. A DC-to-DC voltage converter/auxiliary power
module (APM) 19 may be used to reduce a voltage level of the DC
voltage bus 22 to a lower auxiliary level, e.g., 12-15 VDC, which
in turn may be stored in an auxiliary battery (B.sub.AUX) 44 for
use in energizing low-voltage electrical systems aboard the vehicle
10.
A rotor shaft 65 of the PM machine 16 may be selectively connected
to a load, e.g., an input member 28 of a transmission (T) 30, via
operation of an input clutch 32. The rotor shaft 65 rotates and
thereby delivers an input torque (arrow T.sub.I) to the input
member 28 of the transmission 30 when the PM machine 16 is operated
as an electric traction motor, and/or the PM machine 16 may be
operated as a generator as needed. The motor output torque (arrow
T.sub.m) from the energized PM machine 16 may be directed to the
input member 28 and/or to another load in the form of an output
member 33 of the transmission 30 and a set of drive wheels 34
connected to the output member 33 depending on the configuration of
the electric powertrain 12. Output torque (arrow T.sub.O) from the
transmission 30 may be transmitted to the drive wheels 34 via one
or more drive axles 36.
An optional controller (C) 50 may be used to control ongoing
operation of the PM machine 16 responsive to input signals (arrow
CC.sub.I), doing so via transmission of control signals (arrow
CC.sub.O) to the PM machine 16. For example, the controller 50 may
monitor rotational speed and torque of the PM machine 16 and
actively control a magnetic north-south pole orientation of some or
all of the magnets, e.g., at predetermined torque-speed operating
points or as otherwise needed. The controller 50 may be embodied as
one or more electronic control units having the requisite memory
(M) and a processor (P), as well as other associated hardware and
software, e.g., a clock or timer, input/output circuitry, etc.
Memory (M) may include sufficient amounts of read only memory, for
instance magnetic or optical memory. Instructions embodying a
control method may be programmed as computer-readable instructions
100 into the memory (M) and executed by the processor(s) (P) during
operation of the vehicle 10 to selectively change the magnetic pole
orientation of magnets of the PM machine 16 and thereby optimize
operating efficiency.
FIG. 3A depicts a possible embodiment of the electric machine 16
having a rotor assembly 60 coaxially surrounded/circumscribed by a
stator 62, with the scale of the stator 62 in terms of radial
thickness being reduced in FIG. 3A for the purpose of illustrative
clarity. The rotor assembly 60 is substantially cylindrical in
shape, i.e., generally circular in cross-section and thus forming a
right cylinder with respect to an axis of rotation 66 of the rotor
assembly 60, with possible variations due to manufacturing
tolerances, surface features, and connected components. The stator
62, which axially surrounds and thus fully circumscribes the rotor
assembly 60, is separated from the rotor assembly 60 by a small air
gap 63, such that magnetic flux paths (arrows BB) exist between the
rotor assembly 60 and the stator 62 across the air gap 63. Thus,
the stator 62 is part of a magnetic circuit through which magnetic
flux flows through the flux paths (arrows BB) from the rotor
assembly 60, across an air gap 63, and into the stator 62.
The rotor assembly 60 includes a generally cylindrical rotor 64
splined, integrally formed with, or coupled to the rotor shaft 65
and rotatable therewith about the axis of rotation 66. For
instance, the rotor shaft 65 is splined or otherwise joined to the
rotor 64, with such splines omitted for clarity. The rotor 64
includes a plurality of rotatable magnets 68R, i.e., permanent
magnets. While the particular configuration of the rotatable
magnets 68R may vary within the scope of the disclosure, one
possible embodiment includes affixing a solid bar magnet to a
rotatable plate 61 as shown, or alternatively constructing the
entirety of the rotatable plate 61 as a permanent magnet of a
predetermined field strength.
The rotor 64 may also include an optional set of fixed magnets 68F.
That is, the various permanent magnets used in the construction of
the rotor assembly 60 may be the rotatable magnets 68R,
exclusively, or the rotatable magnets 68R may be used in
conjunction with the fixed magnets 68F, with the fixed magnets 68F
arranged around a circumference of the rotor 64 in alternating
north-south magnetic pole orientations. A respective one of the
rotatable magnets 68R may be positioned adjacent to one or more of
the fixed magnets 68F at a position adjacent to the inner perimeter
wall 69.
For illustrative simplicity, one set of fixed and magnets 68F and
68R are depicted schematically in FIGS. 3A-3C. However, as is well
known in the art, the magnetic poles of a PM machine are
distributed evenly around the circumference of the rotor 64, i.e.,
there are an equal number of north (N) and south (S) poles. Thus,
flanking the depicted fixed magnets 68R and 68F is an identical set
of rotatable and fixed magnets 68R and 68F, with the orientation of
the rotatable magnets 68R possibly having different pole
orientations, i.e., the south (S) magnetic poles oriented toward
the stator 62 or any other orientation depending on the specific
torque-speed operating point of the PM machine 16. As the rotor
assembly 60 rotates about the axis of rotation 66 past a fixed
point on the stator 62, therefore, such a point experiences
alternating north and south poles to establish the requisite
rotational forces.
The PM machine 16 disclosed herein also includes an actuator 70 as
shown schematically in FIG. 3A, with the actuator 70 usable in the
disclosed embodiments herein but omitted for illustrative
simplicity. The actuator 70 ultimately imparts an actuating torque
(arrow T.sub.A) to the rotatable magnets 68R to cause the rotatable
magnets 68R to rotate in a particular direction. The actuator 70,
which may apply the actuating torque (arrow T.sub.A) actively or
passively in different embodiments as set forth below, is
configured to selectively rotate the rotatable magnets 68R, e.g.,
at one or more predetermined operating points of the PM machine 16,
through an angular distance sufficient for changing the above-noted
magnetic pole orientations.
In other words, the magnetic north (N) pole of an illustrative one
of the rotatable magnets 68R may be oriented as shown in FIG. 3A,
e.g., at low-speed/high-torque conditions, and then rotated in FIG.
3B such that the magnetic south (S) poles of the rotatable magnet
68R is oriented toward the stator 62. The angular distance may vary
to provide a desired level of flux weakening, e.g., the rotatable
magnets 68R may be rotated anywhere from 0.degree. to 180.degree.
or beyond. The amount of rotation ultimately modifies the magnetic
flux paths BB between the rotor assembly 60 and the stator 62 to a
corresponding extent.
Such a modification to the flux paths BB is schematically depicted
in FIG. 3B. An example rotation of a particular one of the
rotatable magnets 68R orients its magnetic south (S) pole toward
the stator 62. With fixed magnets 68F oriented as shown, i.e., with
its magnetic north (N) pole oriented toward the stator 62, the
effect is one of flux weakening relative to the orientation of FIG.
3A, and as indicated by the different direction and size of the
arrows corresponding to the flux paths BB. Part of the flux of the
fixed magnets 68F now leaks through the rotatable magnets 68R due
to reverse orientation of the rotatable magnets 68R. As a result, a
level of flux linkage in the stator windings is reduced. Thus, the
rotatable magnets 68R may be rotated when needed, and to the
angular degree required, so as to situationally minimize or
maximize flux linkage with the stator winding of the stator 62.
As an example application within the vehicle 10 of FIG. 1, for
instance, magnetic flux linkage across the air gap 63 of FIGS. 3A
and 3B may be reduced via rotation of the rotatable magnets 68R at
a high speed and low torque of the PM machine 16, e.g., when
operating in the relatively excessive flux area of zone Z in FIG.
2. In terms of actuation, in some embodiments the controller 50 of
FIG. 1 may determine the speed and torque of the PM machine 16,
such as via calculation, measurement, or lookup table, and then
command the actuator 70 to rotate the rotatable magnets 68R when a
threshold operating point is detected, e.g., speeds of the rotor
shaft 65 being higher than a threshold speed and output torque of
the rotor shaft 65 being lower than a threshold torque.
Alternatively, the actuator 70 shown in FIG. 3A may be configured
as a passive actuator 170 to provide passive actuation, i.e.,
without communication with the controller 50. As illustrated in
FIG. 6, the passive actuator 170 may be configured to bias the
rotatable magnets 68R in a first predetermined rotational direction
(arrow EE) below a threshold acceleration force of the rotor 64 of
FIGS. 3A-4B, and to enable rotation of the rotatable magnets 68R in
a second predetermined rotational direction (arrow FF) above the
threshold acceleration force of the rotor 64.
As an example, the passive actuator 170 may be equipped with rotary
spring elements 90 or other passive biasing devices reacting
against a stationary surface 92. The passive actuator 170 in such
an embodiment may be configured with a calibrated return force
sufficient for biasing the rotatable magnets 68R in a predetermined
rotational direction, i.e., the direction indicated by arrows EE.
Such a return force may be predetermined and calibrated based on
the mass of the rotor assembly 60 such that the return force is
overcome in response opposing acceleration forces of a threshold
magnitude in the direction of arrow FF, i.e., rotational forces
generated in a direction opposite to that of the return force.
FIG. 3C is an alternative embodiment to that which is depicted in
FIGS. 3A and 3B. Here, a plurality of torque transfer elements 72
such as arcuate pieces, plates, or other suitable structure may be
disposed within circular openings 74 defined in the rotor 64. The
torque transfer elements 72 may be securely connected to an
optional rotatable flux-shunting element 79 such that rotation of
the torque transfer elements 72 within the circular openings 74 is
sufficient to rotate the rotatable flux-shunting element 79
connected thereto.
The flux-shunting element 79 may be used in some embodiments to
enhance the effects of the above-described selective magnetic pole
variation. For instance, the flux-shunting element 79 may be
embodied as a combined piece of mild steel or other ferromagnetic
metal and plastic or another disparate material, with the different
materials M1 and M2 positioned diametrically opposite each other as
shown. Or, the flux-shunting element 79 may be a single piece of
ferromagnetic material having a non-uniform shape, e.g., as shown
as the flux-shunting element 179 in FIG. 7 and described below.
Aluminum or plastic may be used on one half of the flux-shunting
element 79 of FIG. 3C with mild steel used on the other half, for
example, and/or halves of different relative sizes or geometric
shapes may be used as shown in FIG. 7. Thus, the materials having
high relative magnetic permeability may be used to increase flux
leakage, with the low relative permeability materials rotated into
position to reduce flux leakage as needed.
Changing the angular position of the flux-shunting elements 79
further modifies the magnetic flux paths BB between the rotor 64
and the stator 62. Orientation of the flux-shunting elements 79 may
be achieved by the actuator 70 or a separate actuator, e.g., a cam
plate, or actuated passively in the different embodiments disclosed
herein. Optionally, one of the magnets 68F of a given pole 71 may
be replaced with a rotatable magnet 68R as shown in FIG. 4B, with
flux variation occurring in response to rotation of the
flux-shunting element 79 and/or the rotatable magnet 68R at each
pole 71 around the perimeter of the rotor 64, with one such pole 71
depicted in FIG. 3A for illustrative simplicity.
Referring briefly to FIG. 7, a possible embodiment of the
flux-shunting element 79 of FIG. 3C is a rotatable magnetic
flux-shunting element 179, i.e., a structural element that combines
disparate materials and/or geometry features of the above-described
flux-shunting element 79 with a permanent magnet 75. Thus, the
permanent magnet 75 may be connected to or formed integrally with
the flux-shunting element 79 as part of the same rotatable
structure. Such a rotatable magnetic flux-shunting element 179 may
be attached to the torque transfer elements 72 shown in FIG. 3C,
with flux variation achieved in two manners, i.e., through magnetic
pole variation and by changing the orientation of the magnetically
disparate materials in the flux paths BB.
Referring to FIGS. 4A and 4B, in another possible configuration of
the rotor 64, a respective one of the rotatable magnets 68R is
positioned adjacent to a respective one of the fixed magnets 68F to
form a given pole 71. FIG. 4A also shows an optional embodiment in
which the fixed magnets 68F are replaced with another rotatable
magnet 68R. In either embodiment, the relatively strong flux paths
(BB) of FIG. 4A are weakened by rotation of one or both of the
rotatable magnets 68R making up a given pole 71, with such flux
weakening depicted in FIG. 4B. That is, in the angular position of
FIG. 4B, magnetic flux passes around the rotor 64 through a series
of similarly oriented magnets, with comparatively less of the
magnetic flux passing into the stator 62.
As noted above, rotation of the rotatable magnets 68R may be
triggered by predetermined operating points and commanded by the
controller 50 via the control signals (arrow CC.sub.O) of FIG. 1.
Referring briefly to FIG. 5, for example, a plurality of the torque
transfer elements 72 noted above with reference to FIG. 3C, such as
arcuate pieces, plates, or other suitable structure, may be
disposed within circular openings 74 defined in the rotor 64. The
torque transfer elements 72 may be securely connected to a
respective one of the rotatable magnets 68R such that a rotation of
the torque transfer elements 72 within the circular openings 74 is
sufficient to rotate the rotatable magnets 68R connected
thereto.
An approach for achieving such rotation uses a planetary gear set
80 or other gear arrangement disposed axially adjacent to the rotor
64 and configured to rotate the torque transfer elements 72 in
response to a rotational force imparted to the gear set 80 by the
actuator 70. The gear set 80 may include a sun gear 81 and multiple
pinion gears 82 arranged evenly around the sun gear 81, i.e., a
circle 88 passes through respective center points of the various
pinion gears 82. The sun gear 81 may be rotatably driven by the
actuator 70 of FIG. 3A, e.g., a rotary actuator in this embodiment.
Rotation of the sun gear 81 in the direction of arrow CC would thus
rotate the pinion gears 82 in the opposite direction of arrow DD.
Centering and connecting the torque transfer elements 72
respectively on and to the pinion gears 82 would thus result in
controlled rotation of the rotatable magnets 68R in the direction
of arrow DD. The use of the variable magnetic pole features
described above may be used to tune the performance of the electric
machine 16 across a wide operating range, and to selectively
increase or reduce flux linkage in a magnetic circuit formed
between the rotor assembly 60 and the stator 62 of FIGS. 3A-4B.
FIGS. 8A and 8B depict yet another embodiment in which the gear set
80 of FIG. 5 may be passively rotated using a suitably configured
spring element 96, e.g., a clock spring as depicted, mounted with
respect to the sun gear 81. Flyweights 94, for instance
semicircular weights as shown, are disposed on each pinion gear 82
such that rotation of the gear set 80 occurs in response to
acceleration of the rotor 64 described above. The rotation is
counteracted in the direction of arrow DD by the return force of
the spring element 96, with threshold acceleration forces above the
spring force allowing rotation of the sun gear 81 in the direction
of arrow CC. In other words, the flyweights 94 are configured to
cause rotation of the pinion gears 82 in response to acceleration
forces of the rotor 64 in excess of a predetermined return force of
the spring element 96.
In view of the above-described PM machine 16, those of ordinary
skill in the art will appreciate that a method for controlling flux
in the PM machine 16 is also enabled, with requisite steps of such
method possibly encoded in memory (M) of the controller 50 of FIG.
1 as the instructions 100. For instance, such a method may include
connecting a plurality of the rotatable magnets 68R to the rotor 64
of FIGS. 3A-4B such that each rotatable magnet 68R has a respective
north-south magnetic pole orientation, and then positioning the
stator 62 (see FIG. 3A) with respect the rotor 64 such that the
stator 62 circumscribes the rotor 64 and defines the air gap 63 in
conjunction therewith. In other embodiments, the stator 62 and
rotor 64 may be axially-aligned such that the air gap 63 is defined
in an axial direction between juxtaposed faces of the stator 62 and
rotor 64, as opposed to the radial spacing depicted in the
Figures.
Thereafter, such a method includes selectively rotating the
rotatable magnets 68R, via the actuator 70 in its various
embodiments, at one or more predetermined operating points of the
PM machine 16. Such rotation occurs through an angular distance
sufficient for changing the respective north-south magnetic pole
orientations and thereby modifying the magnetic flux paths BB
between the rotor 64 and the stator 62 across the air gap 63. As
part of such a method, the actuator 70 of FIG. 3A may be used to
rotate the gear set 80 of FIG. 5, which is connected to multiple
torque transfer element 82. Or, the actuator 70 in such a method
may be a passive actuator 170, as shown in FIG. 6, and thus
configured to bias the rotatable magnets 68R as explained
above.
The detailed description and the drawings or figures are supportive
and descriptive of the present teachings, but the scope of the
present teachings is defined solely by the claims. While some of
the best modes and other embodiments for carrying out the present
teachings have been described in detail, various alternative
designs and embodiments exist for practicing the present teachings
defined in the appended claims.
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